The out-of-core nuclear instrumentation device monitors an operation state at a time of a nuclear reactor startup and an output operation by continuously monitoring neutron flux with a neutron detector installed inside a primary shield of a nuclear reactor and transmits an alarm signal and an emergency shutdown signal of the nuclear reactor to protect the nuclear reactor at a time of a nuclear reactor abnormality. The out-of-core nuclear instrumentation device is configured to include the neutron detector which outputs a current value according to a size of the neutron flux by mainly measuring the neutron flux and an out-of-core nuclear instrumentation board which processes a calculation of the output current value and transmits the alarm signal or the like.
In general, since the neutron flux ranging from a stop state of the nuclear reactor outside a furnace to an output operation covers in a very wide range of levels by 10 to 11 digits, a measurement area of the neutron flux is divided into three areas of a neutron source area, an intermediate area, and an output area according to an output level of the furnace and the respective areas are overlapped with each other to continuously monitor the entire area in the out-of-core nuclear instrumentation device. For this reason, structures and functions of the neutron detector and the out-of-core nuclear instrumentation board are different for each area and a detector signal-processing circuit is used for an operational processing of the output area.
Even when an output current of the neutron detector is very small, in order to output a voltage level according to an output level of the furnace and obtain a measured value with high precision, for example, the out-of-core nuclear instrumentation device of PTL 1 includes the detector signal-processing circuit for operational-processing the current value measured by the neutron detector and outputting a state of the neutron flux during operation, in which the detector signal-processing circuit includes a current/voltage conversion part which converts the current value converted by the neutron detector into a voltage value according to the current value and a variable gain amplification part which has an operational amplifier including a current level response-use resistance circuit capable of selecting a gain and a D/A converter adjusting the gain and amplifies the voltage value converted by the current/voltage conversion part. Due to this configuration, it is possible to accurately expand a width of the neutron detector current which can be measured.